Circuit and method for testing multi-device systems

ABSTRACT

A method and system for high speed testing of memories in a multi-device system, where individual devices of the multi-device system are arranged in a serial interconnected configuration. High speed testing is achieved by first writing test pattern data to the memory banks of each device of the multi-device system, followed by local test read-out and comparison of the data in each device. Each device generates local result data representing the absence or presence of a failed bit position in the device. Serial test circuitry in each device compares the local result data with global result data from a previous device. The test circuitry compresses this result of this comparison and provides it to the next device as an updated global result data. Hence, the updated global result data will represent the local result data of all the previous devices.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.12/391,810, filed on Feb. 24, 2009, which is a Continuation of U.S.patent application Ser. No. 11/565,327 filed on Nov. 30, 2006, whichissued as U.S. Pat. No. 7,508,724 on Mar. 24, 2009.

FIELD OF THE INVENTION

The present invention relates generally to the testing of multi-devicesystems. More particularly, the present invention relates to the testingof interconnection multi-device systems.

BACKGROUND OF THE INVENTION

Semiconductor memory devices are important components in presentlyavailable industrial and consumer electronics products. For example,computers, mobile phones, and other portable electronics all rely onsome form of memory for storing data. While many memory devices aretypically available as commodity devices, the need for higherintegration has led to the development of embedded memory, which can beintegrated with systems, such as microcontrollers and other processingcircuits.

Unfortunately, the density of commodity memory cannot match theever-increasing demand for memory. Hence multiple commodity memories areused together to fulfill the system memory requirements. FIG. 1 shows aknown a multi-device memory configuration. Multi-device memory systemscan be implemented as a set of silicon chips grouped together in asingle package (called a multi chip system—MCP), or a multiplicity ofmemory device packages grouped together on a printed circuit board.

FIG. 1 is a block diagram of a prior art multi-device system arranged ina multi-drop, or parallel, configuration. Multi-device system 100includes a memory controller 102 and memory devices 104, 106, 108 and110. The memory controller 102 is the interface between the memorydevices and the system (not shown), while memory devices 104, 106, 108and 110 can be any type of memory device, or system including embeddedmemory. While four memory devices are shown, those skilled in the artwill understand that multi-device system 100 can include any number ofmemory devices, but all sharing a common set of read and write databuses. Hence, any one of the memory devices can provide read datathrough the common set of read databases, when instructed to through thememory controller 102.

Those skilled in the art understand that multi-device systems, such asthe one shown in FIG. 1, is tested at the individual component level andat the system level to ensure robustness of operation. In particular,memory devices are tested at the chip level to ensure that their memorycells are not defective. A defective memory cell is one which does notstore data properly, due to fabrication defects or other defects whichmay occur during fabrication or assembly of the memory device.

One known technique for testing memory devices is the scheme of writingidentical test data into two memory banks and then comparing the readout data from a read operation. Those skilled in the art will understandthat a memory device can include any number of memory banks, but two areused in the present discussion as examples only. If the memory bankswork according to design specifications, the data from one memory bankwill match the data from the other memory bank. Otherwise, there is adefective memory cell and the memory device as a whole is considereddefective. The conceptual approach of this technique is illustrated inFIG. 2. Test pattern data is first written to two memory banks 202 and204 of a memory device. This can be done quickly since the same data canbe written concurrently into both memory banks 202 and 204. Then thedata of both banks 202 and 204 is read out one bit at a time, such thatdata from bank 202 is compared with data from bank 204 by a logicalexclusive OR (XOR) circuit 206. Single bit signal PASS is provided byXOR circuit 206 to indicate the status of the comparison. Differencesbetween the data content of the memory banks will indicate failure of amemory cell. Once identified, defective cells can be replaced withredundant cells using well known redundancy schemes, thereby salvagingan otherwise defective memory device.

Unfortunately, the primary issue with testing is the time required fortesting all the memory cells of every memory device of the system. Inthe previous example, the data of one memory cell of memory bank 202 iscompared with the data of one memory cell of memory bank 204. Thereforelong testing times will result if all memory devices are tested insequence, which adds cost to the production cycle of the memory device.

U.S. Pat. No. 5,579,272 to Uchida, entitled “Semiconductor Memory Devicewith Data Compression test Function and its Testing Method”, addressesthe problem of testing time by using data compression. Data compressioninvolves simultaneously testing several memory cells and representingthe test result with a number of bits that is smaller than the number ofmemory cells tested. The most common approach to data compression is tocompress multiple bits into a single bit output that indicates a failurewhen at least one of the input bits indicates a failure. The drawback ofthis approach is that multiple memory cells are declared faulty in caseswhere only a single cell is truly defective, although it is possible tomitigate this problem, as illustrated by U.S. Pat. No. 5,913,928 toMorzano, entitled “Data Compression Test Mode Independent ofRedundancy.” In Morzano, a failure indication in the compressed testresult is followed by individual testing to isolate the defective cells.

Because of the economies that can be achieved by reducing testing time,various efforts have been made to improve upon the circuitry used tocompare memory cells and compress the test results. U.S. Pat. No.5,926,422 to Haukness entitled “Integrated Circuit Memory Device HavingCurrent-Mode Data Compression Test Mode”, U.S. Pat. Nos. 6,295,618 and6,999,361 to Keeth entitled “Method and Apparatus for Data Compressionin Memory Devices”, and U.S. Pat. No. 6,930,936 to Santin entitled “DataCompression Read Mode for Memory Testing” are all examples of patentsdirected to increasing the speed of the comparison and compressioncircuitry in a memory device.

Using these well-known techniques, it has been possible to maintain lowtesting times for individual memory devices in single chip packagesdespite their increasing size and complexity. It has even been possibleto achieve low testing times in multi-device packages arranged in amulti-drop configuration, such as the one illustrated in FIG. 1. The lowtesting times are still possible in a multi-drop configuration becausethe memory devices inside the chip can be tested in parallel

The difficulties inherent in testing serially interconnected deviceshave long been known, as illustrated by U.S. Pat. No. 5,132,635 toKennedy, entitled “Serial Testing of Removable Circuit Boards on aBackplane Bus”, which illustrates the sequential nature of such testing.The drawback of such sequential testing is that each device is tested insequence, and therefore a multi-device system containing N chips takes Ntimes as long to test as N single-chip packages or a multi-drop packagehaving the same number of chips.

It is noted that most memory devices should be flexible enough to beused, and tested, in both a multi-drop and serially configuredmulti-device memory system. As previously discussed, testing of amulti-drop memory system can be done quickly. However, additionaldedicated circuits may be required for carrying out multi-device systemlevel testing for serial interconnected configurations. Therefore, thecomplexity and size of test circuits should be minimized to keep design,fabrication and testing costs minimized.

It is, therefore, desirable to provide a testing scheme for amulti-device memory system arranged in a serial interconnectedconfiguration that can achieve high testing speeds. It is furtherdesirable to minimize the amount of additional test circuits requiredfor implementing the testing scheme.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous serially interconnected multi-chip ormulti-device systems by compressing the test results of each device inthe serial interconnected configuration, with the test results of theprevious chip in the system.

In a first aspect, the present invention provides test circuit for amemory device having a serial input connection, and a serial outputconnection. The test circuit includes a local comparison circuit and acompression circuit. The local comparison circuit compares data of atleast two memory cells with each other, and provides local result datacorresponding to a fail status if one of the at least two memory cellsis defective. The compression circuit logically OR's the local resultdata with a global result data received from the serial inputconnection, for providing updated global result data to the serialoutput connection. The updated global result data corresponds to thefail status if at least one of the local result data and the globalresult data corresponds to the fail status.

According to an embodiment of the present aspect, the test circuitfurther includes a first stage test path selector for selectivelypassing one of the local result data and local memory data, the firststage test path selector passing the local result data to thecompression circuit in response to a test mode signal. The test circuitcan further include a second stage test path selector for selectivelypassing one of the local result data and global memory data, the secondstage test path selector passing the local result data to the serialoutput connection in response to the test mode signal. The localcomparison circuit can include logic circuitry for receiving the datafrom the at least two memory cells, the logic circuitry being configuredto provide the local result data corresponding to a fail status when thedata mismatch a predetermined pattern. The local comparison circuit caninclude an exclusive OR circuit for receiving the data from the at leasttwo memory cells, the local result data being provided by the exclusiveOR circuit.

In further aspects of the present embodiment, the compression circuitincludes an OR circuit for receiving the global result data and thelocal result data, the OR circuit providing the updated global resultdata. The memory device includes a device selector for selectivelypassing one of the local memory data and the global memory data as thememory data, the local data being received from one of the at least twomemory banks and the global data being received from the serial inputconnection. Each of the at least two memory cells can be located indifferent segregated memory areas or can be located in the samesegregated memory area. The memory device includes a parallel to serialregister for receiving and storing the local result data, the parallelto serial register being at least n-bits wide for receiving n-bit widelocal result data in parallel and for serially outputting one bit of then-bit wide local result data synchronously to a clock. The memory devicecan include a flip-flop circuit for receiving the global result datareceived from the serial input connection and for providing one bit ofthe global result data synchronously to the clock.

In a second aspect, the present invention provides memory system. Thesystem includes a first memory device operable in a test mode forproviding a single-bit global result data, the single-bit global resultdata corresponding to a first memory device cell defect, and a secondmemory device operable in the test mode for receiving the single-bitglobal result data, the second memory device providing an updatedsingle-bit global result data corresponding to at least the one firstmemory device cell defect and a second memory device cell defect.According to an embodiment of the present aspect, the system furtherincludes a memory controller for providing default pass data and forreceiving the updated single-bit global result data, during the testmode of operation. The memory controller can include test circuitry,which can include a providing circuit for providing default pass dataand a receiving circuit for receiving the updated single-bit globalresult data, during the test mode of operation. The providing circuitand the receiving circuit can be dedicated circuits.

According to another embodiment of the present aspect, the first memorydevice includes a first local comparison circuit for detecting the firstmemory device cell defect and for providing first local result data, anda first compression circuit for providing the single-bit global resultdata corresponding to a status of the first local result data. In yetanother embodiment, the second memory device includes a second localcomparison circuit for detecting the second memory device cell defectand for providing second local result data, and a second compressioncircuit for executing a logical OR function upon the single-bit globalresult data from the first memory device and the second local resultdata. The second compression circuit provides an updated single-bitglobal result data corresponding to an output of the logical ORfunction. The first local comparison circuit can include a firstexclusive OR circuit for detecting the first memory device cell defect,and the second local comparison circuit can include a second exclusiveOR circuit for detecting the second memory device cell defect. The firstcompression circuit can include a first OR circuit for receiving thedefault pass data and the first local result data, and the secondcompression circuit can include a second OR circuit for receiving thesingle-bit global result data and the second local result data.

In a third aspect, the present invention provides a method for testing amemory system. The method includes a) comparing at least two memorycells in a memory device for detecting one memory cell defect; b)providing a local result data corresponding to a fail status when theone memory cell defect is detected, the local result data correspondingto a pass status in an absence of the one memory cell defect; c)comparing the local result data to global result data, the global resultdata corresponding to one of the fail status and the pass status; and,d) providing updated global result data from the memory device, theupdated global result data having a status corresponding to the failstatus if at least one of the global result data and the local resultdata corresponds to the fail status.

Embodiments of the present aspect are as follows. The local result data,the global result data and the updated global result data are single-bitsignals. The step of comparing at least two memory cells is preceded bywriting identical test data to all the memory cells of the memorydevice. The step of comparing at least two memory cells includes readingdata corresponding to the at least two memory cells, and the step ofproviding includes executing an exclusive OR operation upon the data ofthe at least two memory cells for providing the local result data. Thestep of comparing includes receiving the global result data externallyfrom the memory device. The step of providing the updated global resultdata includes logically OR'ing the global result data with the localresult data for providing the updated global result data. The globalresult data is provided by a previous memory device or by a memorycontroller, and if by a memory controller, the global result data ispreset to have the pass status. The updated global result data isprovided to a subsequent memory device, a memory controller or to atester system.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 is a block diagram of a prior art multi-device package arrangedin a multi-drop configuration;

FIG. 2 is a block diagram of the prior art memory device testing scheme;

FIG. 3 a is a block diagram of a multi-device package arranged in aserial interconnected configuration, to which an embodiment of thepresent invention is applicable;

FIG. 3 b is a block diagram of a memory device having compression testcircuitry according to an embodiment of the present invention;

FIG. 4 a is a block diagram of serially interconnected testable memorydevices according to an embodiment of the present invention;

FIG. 4 b is a block diagram of serially interconnected testable memorydevices according to another embodiment of the present invention.

FIG. 5 is a flow chart of a memory system device testing method,according to an embodiment of the present invention;

FIG. 6 is a circuit schematic of output path circuitry and compressiontest circuitry for a memory device, according to an embodiment of thepresent invention;

FIG. 7 a is a block diagram of serially interconnected memory deviceshaving the compression test circuitry shown in FIG. 6, illustrating theflow of information between memory devices;

FIG. 7 b is a timing diagram showing the operation of the system of FIG.7 a;

FIG. 8 a is a block diagram showing an example testing method operationaccording to an embodiment of the present invention when there are nodefects in the memory devices;

FIG. 8 b is a block diagram showing an example testing method operationaccording to an embodiment of the present invention when there aredefects in the memory devices; and

FIG. 9 is a block diagram showing the local comparator circuitconfigured for a single memory bank device, according to an embodimentof the present invention.

DETAILED DESCRIPTION

Generally, the present invention provides a method and system for highspeed testing of memories in a multi-device system, where individualdevices of the multi-device system are arranged in a seriallyinterconnected configuration. The preferred embodiments of the inventionare directed to a final testing of multi-device systems that have beenassembled or packaged. At this point, the individual devices havealready been subjected to testing, validation, and in those cases wherenecessary, redundancy implementation. However, mechanical and electricalstress involved during the handling and packaging of the multi-devicesystem can cause further failures. Therefore, since the devices areassembled or packaged, any memory bit failure discovered in anyindividual device renders the entire system defective.

From this point forward, references to multi-device systems will includemulti-device systems integrated within a single package, systems havingmultiple discrete devices interconnected on a printed circuit board, ormultiple devices integrated within the same semiconductor chip. Memorybanks, memory areas and memory blocks are intended to be considered atype of segregated memory, where the segregated portions are addressablefor read and write operations.

FIG. 3 a is a block diagram of a multi-device package arranged in aserial interconnection to which an embodiment of the present inventionis applicable. Those skilled in the art will understand that a serialinterconnection refers to a physical arrangement of devices where two ormore devices are serially interconnected to each other. Multi-devicesystem 120 includes a memory controller 122 and memory devices 124, 126,128 and 130.

The components shown in FIG. 3 a can be identical to those previouslydescribed for the multi-device system 100 of FIG. 1. In FIG. 3 a, thememory devices are connected serially, such that the output of onememory device is provided to an input of the next memory device. Theinput of the first memory device and the output of the last memorydevice are connected to the memory controller 122. Hence, bothinstructions and data are transferred from memory device to memorydevice. The number of memory devices can be any number. Theconfiguration of the serial interconnection of multiple devices isdisclosed by U.S. patent application Ser. No. 11/324,023 filed Dec. 30,2005 assigned to the common assignee, the contents of which are entirelyincorporated herein by reference.

According to an embodiment of the present invention, simultaneoustesting is possible in a multi-device package arranged in a serialinterconnection, regardless of the serial nature of the configuration inwhich the individual memory devices are physically or logically arrangedwithin the package. In a case where the location of the defect in theserial interconnection can be identified, it is preferable to store theaddresses of fail locations in a test system or a memory controller thathas a test capability. Individual tests may follow thereafter.

In an embodiment of the present invention, high speed testing isachieved by first writing test pattern data to the memory banks of eachdevice of the multi-device system, followed by local test read-out andcomparison of the data in each device. Each device generates localresult data representing the absence or presence of a failed bitposition, or defect, in the device. Serial test circuitry in each devicecompares the local result data with global result data from a previousdevice. The test circuitry compresses this result of this comparison andprovides it to the next device as an updated global result data. Hence,the updated global result data will represent the local result data ofall the previous devices.

According to the embodiments of the present invention, a final globalresult data provided from the last device in the serial interconnectioncorresponding to a “pass”, indicates there are no defects in any of thememory devices. On the other hand, a final global result datacorresponding to a “fail” indicates that there is at least one memorycell amongst the memory banks of all the devices that is defective. Inpreferred embodiments of the present invention, each local result dataand each global result data is a single-bit data signal. Since only asingle bit of data propagates from device to device, the test logic issimple and does not require significant silicon area to implement.

FIG. 3 b is a block diagram of a memory device according to anembodiment of the present invention. More specifically, memory device300 is shown to include a memory circuit 302 including a local memorytest circuit, which can be of the type illustrated in FIG. 2, but mayalso be any other logic circuit capable of indicating whether one of itsmemory banks contains defects. In the present embodiment, memory circuit302 includes a first memory bank 304 and a second memory bank 306, bothcoupled to a test circuit including a XOR circuit 308. The memory device300 further includes an input terminal 310 for connection to a previousdevice (not shown), a compression circuit 312, and an output terminal314 for connection to a next device (not shown). The compression circuit312 is illustrated as a simple OR logic circuit, but can be implementedwith any logic circuit, or circuits, capable of combining the signalsreceived from input terminal 310 and memory circuit 302. The inputterminal 310 receives single-bit global result data from the outputterminal 314 of a previous device in the system. During a test mode ofoperation, the compression circuit 312 logically compresses, orcombines, the global result data received from the input terminal 310with local result data from a local comparison circuit, presentlyembodied as XOR circuit 308 in the memory circuit 302.

If the comparison between the data from the input terminal 310 and theXOR circuit 308 indicates a failure, the compression circuit 312 willprovide a “fail” signal via the output terminal 314. Otherwise, a “pass”signal is provided. According to the preferred embodiment, the “pass”and “fail” status is represented by a single bit of data. The signal,called updated global result data, transmitted from the output terminal314 is then provided to the input terminal 310 of the next memory device300 in the system. Therefore, each compression circuit 312 updates theglobal result data received from the input terminal 310 with the localresult data, to provide an updated global result data. Detailedembodiments of the test circuitry will be discussed later.

FIG. 4 a is a block diagram of a multi-device system including seriallyinterconnected memory devices. In the present embodiment, multi-devicesystem 400 includes a memory controller 402, and memory devices 404,406, 408 and 410. Each memory device 404, 406, 408 and 410 is preferablyimplemented with the memory device 300 of FIG. 3 b. The memorycontroller 402 includes test circuitry therein. The test circuitryincludes a test signal provider 418 and a test result receiver 420. Thetest signal provider 418 provides a test signal containing a testpattern to the first memory device. The test result receiver 420receives a test signal containing a test result data from the lastmemory device. Those skilled in the art will understand that memorycontroller 402 can be any control logic responsible for controllingoperation of the memory devices.

During normal operation, the memory controller 402 provides write datato a specific memory device and requests read data from a specificmemory device, by providing a device enable signal. In order to simplifythe drawing, the connections for communicating command, address andread/write data are not shown. Hence only the addressed device, via adevice identifier is enabled for acting upon the command and the addressinformation, while the preceding memory devices allow the command andthe address data to flow through, and the following memory devices allowread data to flow through and back to the memory controller 402.

The test mode operation of the multi-device system 400 according to anembodiment of the present invention is now described in more detail. Inthe test mode, memory controller 402 sends a write command with testpattern data to the first memory device 404. In response to a uniquecode, value or number, all the devices will be enabled for acting uponthe command, address and write data from memory controller 402. Theunique code, value or number is not used by the system for any otherfunction, or a test command. Since the memory devices are connected inseries with each other, the command, address and write data aretransferred from device to device, with a preset latency associated withclocking the data through each memory device. Once the write data hasbeen written into the memory banks of all the memory devices 404, 406,408 and 410, memory testing according to the embodiments of theinvention can proceed. Alternately, the unique code, value or number canbe provided to the memory devices. Thereafter, each memory device willread the stored preset test data to determine if any defects existlocally. The local result data, representing the result of the localcomparison, is compared with previous global result data, and fedforward to the next memory device where the same comparison isperformed.

In FIG. 4 a, since memory device 404 is the first memory device in theserial interconnection, its input terminal will receive default “pass”data from memory controller 402. For example, the default pass data canpreset to be one or more logic ‘0’ bits. Memory device 404 provides fromits output, updated global result data 412 to memory device 406. Memorydevice 406 then combines the previous global result data received frommemory device 404 with its local result data to generate an updatedglobal result data 414. If the local result data from memory device 404or the local result data from memory device 406 corresponds to a “fail”,the updated global result data 412 and 414 will also indicate a “fail”.Memory device 406 transmits its updated global result data 414 to memorydevice 408, which repeats the previously described internal comparisons.Eventually the updated global result data reaches memory device 410, thefinal memory device in the serial interconnection, which provides afinal global result data 416 back to memory controller 402. The finalglobal result data 416 generated by memory device 410 will correspond toa “fail” status if even one memory cell of any device in the serialinterconnection is defective.

FIG. 4 b shows serially interconnected multi-memory devices according toanother embodiment of the present invention. This example is similar tothat of FIG. 4 a, except that the test signal provider 418 and the testsignal receiver 420 can be implemented as logic circuits external to amemory controller, or similar type of controller. Referring to FIG. 4 b,a test signal provider 422 and a test result receiver/determinator 424are configured separately from each other. In this example, four memorydevices are serially interconnected. Serially interconnected devices404, 406, 408 and 410 are connected to the test signal provider 422 andthe test result receiver/determinator 424. The test signal provider 422receiver/determinator 424 performs the same function of the test signalprovider 418. The test result receiver/determinator 424 performs thesame function of the test signal provider 418 and also performs thefunction of determining “failure” or “pass” from the final global resultdata 416 generated by memory device 410.

FIG. 5 is a flow chart showing a method of testing a seriallyinterconnected multi-device memory system, according to an embodiment ofthe present invention. For the purposes of the present example, eachmemory device is presumed to have two memory banks, although thoseskilled in the art will understand that any number of memory banks canbe present. Furthermore, those skilled in the art will understand thatthe embodiments of the present invention are applicable to devicesorganized to have a single memory array. A detailed description of thememory testing scheme applied to single memory array devices will bedescribed with respect to FIG. 9.

The test method starts at step 500 by writing preset test data to everybank in each memory device. In order to minimize the time required forwriting all the memory banks of each memory device, the preset test datais preferably the same in each memory bank. One example of preset testdata that may be used is a checkerboard pattern, or all logic “0” dataor all logic “1” data. According to an embodiment of the presentinvention, a write command can be provided to all the memory deviceswith a unique code, value or number which does not correspond to anydevice in the system. In response to this unique code, a virtual testmode is entered and all the memory blocks of each memory device are setto receive the preset test data at the same time. Such control can beprovided by a command decoder of the device, as should be well known tothose skilled in the art. Otherwise, a specific test command can beprovided to each of the memory devices to place the system in the testmode of operation.

Once all the memory banks have been written with the preset test data, afirst bit position i of the two memory banks of the first memory devicej, are read at step 502. Variables i and j start at 0 at step 502. Testcircuitry, such as XOR circuit 308 of FIG. 3 b for example, is then usedfor generating a single-bit local result data representing the pass orfail status of bit position i=0 of the two memory banks. At step 504,the local result data for bit position i=0 is compared to bit positioni=0 of a previous global result data from a preceding memory device j−1.The first memory device j=0 will receive default “pass” result datafrommemory controller as there is no preceding memory device in the system.This comparison can be executed by OR circuit 312 of FIG. 3 b forexample. The output of this comparison is provided as an updated globalresult data for the current memory device j at step 506.

A determination of the current memory device j as being the last memorydevice in the system is made at step 508. If the current memory device jis not the last memory device in the system, then the next memory deviceexecutes steps 502 to 508 for bit position i=0. This is expressed in thepresent method by incrementing j (j=j+1) at step 510 and looping back tostep 502. Steps 502 to 510 repeat until a final updated global resultdata is provided by the last memory device, at which point the methodproceeds to step 512, indicating that the testing for bit position i=0for all the memory banks has been completed. A determination of thepass/fail status of the final updated global result data is made at step512. If the bit status corresponds to a fail, then the method ends atstep 514 and the system as a whole is deemed as failing the test,thereby obviating the need to test further bits of the memory banks. Onthe other hand, if the bit status corresponds to a pass, then the methodcontinues to step 516, where a determination of the last bit position ofthe memory banks is made. If the final updated global result datacorresponds to the last bit position, then the method ends at step 518and the system as a whole has passed the test.

Otherwise, there are more bit positions remaining to be tested. Themethod then loops back to step 502 where the next bit position of thememory banks are to be tested, starting with the first memory device ofthe system. This is expressed in the present method by incrementing i(i=i+1) at step 520, resetting j=0 at step 522, and looping back to step502. If there are no defects in any of the memory banks, the methodrepeats the 502 to 510 loop and the 502 to 522 loop to test every bitposition in the memory banks of all the memory devices.

FIG. 6 is a detailed schematic embodiment of the memory device 300illustrated in FIG. 3 b. Memory device 600 includes standard output pathcircuits and a test circuit 602. The standard output path circuits canbe further subdivided into a flow-through output path for receivingread/write data from a previous memory device, a local output path forproviding local read data, and a shared output driver path for drivingdata received from either of the flow-through output path and the localoutput path. The flow-through output path includes an input terminal604, an input buffer 606, and a D-type flip-flop 608. The local outputpath includes a memory bank selector 610 and a parallel to serialregister 612 of n-bits. The shared output driver path includes a deviceselector 614, an output buffer 616, and a serial output terminal 618.

The general operation of the output path circuits should be well knownto those skilled in the art. By example, the memory bank selector 610allows individual memory banks to be selected for data read out when thememory device is functioning in a normal mode of operation. Depending ondevice selection signal D_SEL (related to the device ID number), deviceselector 614 will pass local data provided by the local output path orglobal data provided from the flow-through output path. Generally, thelocal data and the global data passed from device selector 614 areconsidered memory data during normal operating modes. It is noted thatthe D-type flip-flop and parallel to serial register 612 are clockedwith the same signal CLK in order to ensure that the output data isproperly synchronized with the system regardless the source of the data.In flow-through operation, data is therefore subjected to a single clockcycle latency as it is transferred from one memory device to another.

In FIG. 6, the local output path and the shared output driver path areadapted with minimal logic and circuits for enabling the serial testscheme embodiment of the present invention. The additional serial testcircuit 602 includes a local comparator circuit 620, a first stage testpath selector 622 of n-bits, a compression circuit 624 also referred toas a global comparison circuit, and a second stage test path selector626. The compression circuit 624 is presently implemented as an ORcircuit by example, however, those skilled in the art will understandthat any functionally equivalent circuit can be used. The localcomparator circuit 620 is implemented as an XOR circuit by example,although any functionally equivalent circuit can be used as long as itcan logically detect the presence or absence of defects by comparingmemory cell contents. More specifically, any logic circuitry can be usedto determine if the memory cell contents mismatch a predeterminedpattern, or other expected output, based on the written test pattern.XOR circuit 620 has a first input for receiving a data bit from one bank(Bank 0) and a second input for receiving a data bit from another bank(Bank 1). In the present example, as long as the two bits from Bank 0and Bank 1 are logically the same, XOR circuit 620 will output a logic“0” result, which corresponds to a “pass” result.

The first stage test path selector 622 is used to selectively pass theoutput of the memory bank selector 610 which provides local memory datafrom the memory array, or the output of XOR circuit 620 in response toselection signal TEST_MODE. In the test mode of operation, thesingle-bit output of XOR circuit 620 is passed to the parallel to serialregister 612. This output is the local result data which is to besubsequently compared to the previous global result data from thepreceding memory device. In the present example, n-bits from Bank 0 andBank 1 can be concurrently compared with each other, hence there are nlocal result data bits that are stored in the parallel to serialregister 612. Accordingly, memory bank selector 610, local comparatorcircuit 620 and first stage test path selector 622 are configured toaccommodate an n-bit wide word.

Once loaded, the parallel to serial register 612 provides one single-bitlocal comparison result to compression circuit 624 on each clock cycle,where it is compared to a corresponding bit of the previous globalresult data received from D-type flip-flop 608. By example, compressioncircuit 624 is implemented as an OR logic circuit having a first inputconnected to the output of parallel to serial register 612 and a secondinput connected to the output of D-type flip-flop 608. The single-bitoutput of OR logic circuit 624 combines the previous global result datafrom a preceding memory device with the local result data contained inthe parallel to serial register 612. The second stage test path selector626 selectively passes the output of OR logic circuit 624 in response tosignal TEST_MODE. In the test mode of operation, the output of OR logiccircuit 624 is passed to output buffer 616 for transmission to the nextmemory device, or the memory controller. In a normal mode of operation,second stage test path selector 626 passes global memory data receivedat the device selector 614.

A discussion of the operation of the standard output path circuits andthe additional serial test circuit 602 of memory device 600 now follows.During the test mode of operation, control signal TEST_MODE is set suchthat the first stage test path selector 622 passes data from its “1”input terminal, and the second stage test path selector 626 passes datafrom its “1” input terminal. The first stage test path selector 622passes the result of the comparison of Bank 0 and Bank 1 performed bylocal comparator circuit 620 to the parallel to serial register 612. Inthe present example, 8 bits of data from each bank can be simultaneouslyread out and compared with each other. Hence there are eight comparisonresults loaded into parallel to serial register 612. On each activeclock edge, D-type flip-flop 608 and parallel to serial register 612transmit single bits of data to compression circuit 624, which combinesthem and outputs a compressed result. The compressed result passesthrough the second stage test path selector 626 and is sent to the nextmemory device in the serial interconnection via the output buffer 616and the output terminal 618. It is noted that the state of selectionsignal D_SEL is not relevant here since the output of device selector614 is blocked at the second stage test path selector 626.

FIG. 7 a is a block diagram of serial interconnected memory devices 700,702, 704 and 706 for illustrating the flow of information betweendevices. Each memory device includes the previously described serialtest circuit embodiment shown in FIG. 6. Each memory device has thefollowing input ports: a serial input port SI, an input enable port IPE,an output enable port OPE, and a clock input port. Each memory devicehas the following output ports: a serial output port SOP, an inputenable port IPEQ and an output enable port OPEQ. Output ports IPEQ andOPEQ are flow-through ports which transmit the signals received on theIPE and OPE input ports respectively, to the next device in the system.

FIG. 7 b is a timing diagram showing the operation of the seriallyinterconnected multi-device system of FIG. 7 a using the memory testingscheme embodiment of the invention. The timing diagram plots the signaltraces for the system clock CLK, the ports SIP, IPE and OPE for memorydevice 700 “Device 0”, labeled SIP[0], IPE[0] and OPE[0] respectively,and the serial output ports for all four devices, labeled SOP[0],SOP[1], SOP[2] and SOP[3]. It is assumed that the test write data hasbeen written to all the memory banks of memory device 700, 702, 704 and706, and the system of memory devices is ready to be tested.

As can be seen from FIG. 7 b, memory device 700 receives an input enablesignal IPE in the testing operation. As previously mentioned, the uniquecode, value or number will enable each device to act on the controlsignals. The input enable signal IPE[0] is transferred through memorydevices 700 through 706, enabling each in turn. Next, the memory device700 receives a test read command through serial input port SIP[0], whichcan include setting selection signal TEST_MODE to the necessary teststate. For example, signal TEST_MODE can be set to the test state inresponse to the unique code accompanying the read command, or can beexplicitly set when the device enters a test mode of operation. Aspreviously mentioned, the appropriate write circuitry can be activatedfor enabling simultaneous writing of the test data into the memory cellsof one or more memory banks in response to the unique code. Although itis not illustrated in FIG. 7 a, the latching circuitry in each of thememory devices 700, 702, 704 and 706 will cause a delay of one clockcycle between the beginning of the read operation in one memory deviceand the beginning of the read operation in the next memory device in thesystem. This is referred to as a one clock cycle latency. In response tothe read test command, each device will commence local read out andcomparison of data between the cells of its memory banks, and load itsparallel to serial register 612 with local comparison results.

Once a sufficient number of clock cycles has elapsed for completing thelocal comparison, an output enable signal OPE is received by memorydevice 700, which is transferred through memory devices 700, 702, 704and 706. The first memory device 700 in the system receives the outputenable signal OPE, and default “pass” data from the memory controller.This default “pass” data simulates a previous global result data havinga “pass” status from a preceding memory device, and is used by ORcircuit 624 for the comparison with the local result data. In responseto the OPE signal and with a first rising edge of the system clock CLK,a local result data bit from the parallel to serial register 612 ofmemory device 700 is provided to its corresponding OR circuit 624. Alsoat the first rising edge, the D-type flip flop circuit of memory device700 will provide one bit of the default “pass” data to OR circuit 624.In FIG. 7 b, the resulting first updated global result data bit isprovided on SOP[0] at clock edge 708.

One clock cycle later, memory device 702 latches this first updatedglobal result data bit from SOP[0], and then compares it to a first bitfrom its own parallel to serial register 612 to generate a secondupdated global result data bit from SOP[1]. If the data bit from eitherSOP[0] or the local result data from memory device 702 is indicative ofa defect, then SOP[1] will be indicative of a defect as well. Thisprocess continues as memory device 704 receives the second updatedglobal result data bit from SOP[1] of memory device 702 and combines itwith its own local result data to generate a third updated global resultdata bit from SOP[2]. Likewise, memory device 706 will provide a fourthand final updated global result data bit from SOP[3], which is returnedto the memory controller. As can be seen from FIG. 7 b, the latchingcircuitry in each of the memory devices 700, 702, 704 and 706 causes adelay of one clock cycle between the beginning of the output in eachmemory device and the beginning of the output from the next memorydevice in the system. The final updated global result data bit from thelast memory device 706 in the system is a compressed representation ofthe comparison results from all of the devices in the multi-devicesystem.

Therefore, the first bit of the final global result data representingthe pass/fail status of one memory location of all the memory devices isprovided after four clock cycles. For each clock cycle thereafter,successive final global result data bits are provided. The memorycontroller can be configured to halt the testing process as soon as thefirst “fail” bit is received, since any single bit failure in any memorydevice will render the entire system defective.

FIGS. 8 a and 8 b conceptually illustrate example operations of threeserially interconnected memory devices according to the embodiment thepresent invention. Each memory device contains two memory banks labeledBank 0 and Bank 1, an OR circuit, and an XOR circuit. The XOR circuitcorresponds to the local comparator circuit 620 of FIG. 6 thatdetermines whether the contents of the two memory banks are the same.The OR circuit corresponds to compression circuit 624 of FIG. 6 thatcombines the local result data produced by the XOR circuit with theupdated global result data of the previous memory device in the system.The output of the OR circuit is an updated global result data. In FIGS.8 a and 8 b, 8 bits of Bank 0 and Bank 1 are tested at the same time.

FIG. 8 a shows an example situation where there are no defects in any ofthe 16 memory cells of memory devices 800, 802 and 804. For the purposesof illustration, the same test data 00101101 has been written intomemory banks Bank 0 and Bank 1 of devices 800, 802 and 804. Since thememory banks of devices 800, 802 and 804 are not defective, every memorybank will retain the same byte of information. Therefore, during thelocal comparison operation performed by each device, the resultgenerated by the XOR circuit of each memory device will be 00000000.Memory device 800 combines the test result generated by its XOR gatewith the default “pass” data from the memory controller, since device800 is the first device in the system. Since there are no detecteddefects, the final compressed data result will be a string of “0” logicstates, indicative of a “pass” condition for each memory cell positionof all the memory devices.

FIG. 8 b shows an example situation where there is one defect in Bank 0of memory device 806, and one defect in Bank 1 of memory device 808.More specifically, after the test data 00101101 is written to all banksof the memory devices, the third bit position from the left in Bank 0 ofmemory device 806 has switched states from the written “1” logic stateto a “0” logic state. Similarly, the eighth bit position from the leftin Bank 1 of memory device 808 has switched states from the written “1”logic state to a “0” logic state. Thus, when the XOR circuit of memorydevice 806 compares eight bits of Bank 0 to the corresponding eight bitsof Bank 1, it will generate the following output byte of: 00100000.Similarly, when the XOR circuit of memory device 808 compares eight bitsof Bank 0 to the corresponding eight bits of Bank 1, it will generatethe following output byte of 00000001. The “1” logic bit indicates a“fail” for the corresponding bit position of one of the two memorybanks. Because memory device 806 is the first memory device in thesystem, its OR circuit merely transmits the output of its XOR circuit tomemory device 808. Memory device 808 then combines the updated globalresult data received from memory device 806 with the local result datagenerated by its XOR circuit. The updated global result data from memorydevice 808 now contains two error indications: 00100001. The output ofthe OR circuit of device 808 is next transmitted to device 810, whichcombines it with its local result data. Because the data received fromdevice 808 contains error indications, the updated and final compresseddata result from the OR gate in device 810 will contain the same errorindications, even though device 810 has no defects.

The previously described embodiments have been illustrated using memorydevices having two memory banks. In alternate embodiments, the memorydevices can include a plurality of memory banks, and the XOR circuit forcomparing the data from the memory banks can be configured to receiveone bit of data from each of the plurality of memory banks. Thesingle-bit XOR circuit output will still provide an indication of atleast one defect in the plurality of memory banks as the circuit isidentifying at least one defect amongst all the memory banks. In amodification of FIG. 6 for example, memory bank selector 610 can beconfigured to receive data from four banks, and XOR circuit 620 wouldthen be correspondingly configured to receive the same data from thefour banks.

Alternately, the OR circuit for comparing the previous global resultdata of a prior memory device can be configured to receive local resultdata bits from more than one parallel to serial register at the sametime. In a modification of FIG. 6 for example, elements 610, 620, 622and 612 can be duplicated for each pair of memory banks in the memorydevice.

The previously described multi-device system has been described usingmemory devices. However, the embodiments of the present invention can beused in any type of device having embedded memory organized into memorybanks. The previously described embodiments have been shown in use withdevices having at least two memory banks. Those skilled in the art willunderstand that the shown circuits can be scaled to accommodateadditional memory banks. According to another embodiment of theinvention, devices having a single memory array can be tested in thesame way.

FIG. 9 is a block diagram showing a circuit configuration of the localcomparator circuit relative to a single memory bank device. Memorydevice 900 is configured to have a single memory bank 902 having rows ofwordlines and columns of bitlines. Connected to the bitlines are columnaccess circuits 904, which can include bitline sense amplifiers forsensing the voltage levels of the bitlines and column select devices forcoupling the sensed voltages onto read databus lines 906. The readdatabus lines 906 are labeled DB0 to DB7. In the presently shownexample, the memory device 900 is configured to provide eight bits ofdata at the same time onto the read databus lines 906 in each readcycle. In the present embodiment, the local comparator circuit includesone exclusive OR (XOR) circuit for comparing data on two different readdatabus lines. Hence in the present example of eight read databus lines906, there are a total of four XOR circuits 908, 910, 912 and 914. Thelocal result data from each of the XOR circuits 908 to 914, labeledLRD0, LRD1, LRD2 and LRD3, can then be provided to downstream circuitssimilar to those shown in the embodiment of FIG. 6 for comparison to theglobal result data from the previous memory device.

As such, devices having unitary memory arrays or memory arrayssegregated into two or more distinct memory blocks or banks aretestable. A segregated memory array is one where its portions areaddressable for read or write operations.

The previously shown embodiments have been shown using a single stage ofXOR circuits for performing the local comparison of the memory cells ina device. In the embodiment of FIG. 6, there are n XOR circuits 620 forproviding n local comparison data bits in parallel. In the embodiment ofFIG. 9, there are 4 local comparison data which are provided inparallel. In an alternate embodiment, the local comparison circuits canbe organized into stages, such that one single output from a final localcomparison circuit can be provided. This final output will thenrepresent the local comparison results of all the memory cells beingtested during one read cycle. In a modification of the embodiment ofFIG. 9 for example, the outputs LRD0 and LRD1 can be provided to a fifthXOR gate while the outputs LRD2 and LRD3 can be provided to a sixth XORgate. Then the outputs of the fifth and sixth XOR gates can be providedto a seventh and final XOR gate.

In the preceding description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe embodiments of the invention. However, it will be apparent to oneskilled in the art that these specific details are not required in orderto practice the invention. In other instances, well-known electricalstructures and circuits are shown in block diagram form in order not toobscure the invention. For example, specific details are not provided asto whether the embodiments of the invention described herein areimplemented as a software routine, hardware circuit, firmware, or acombination thereof. In circuit structures, devices and elements may bedirectly or indirectly connected to or coupled with others.

Embodiments of the invention can be represented as a software productstored in a machine-readable medium (also referred to as acomputer-readable medium, a processor-readable medium, or a computerusable medium having a computer-readable program code embodied therein).The machine-readable medium can be any suitable tangible medium,including magnetic, optical, or electrical storage medium including adiskette, compact disk read only memory (CD-ROM), memory device(volatile or non-volatile), or similar storage mechanism. Themachine-readable medium can contain various sets of instructions, codesequences, configuration information, or other data, which, whenexecuted, cause a processor to perform steps in a method according to anembodiment of the invention. Those of ordinary skill in the art willappreciate that other instructions and operations necessary to implementthe described invention can also be stored on the machine-readablemedium. Software running from the machine-readable medium can interfacewith circuitry to perform the described tasks.

The above-described embodiments of the invention are intended to beexamples only. Alterations, modifications and variations can be effectedto the particular embodiments by those of skill in the art withoutdeparting from the scope of the invention, which is defined solely bythe claims appended hereto.

1. A memory system comprising: at least two memory devices operable in atest mode, where a last memory device of the at least two memory devicesprovides a single-bit global result data indicating a cell defect in anyof the at least two memory devices.